The present invention relates to an injection molding machine with a stationary frame which guides rods of a movable frame for movement in longitudinal direction. The end of the rods which project beyond spaced upright plates of the stationary frame respectively carry fixed thereto a pair of movable plates one of which carries fixed thereto a half of a split mold opposite the other half which is fixed to the stationary plate adjacent this one movable plate. A unit including a pressure plate fixed to the other stationary plate is mounted between the latter and the other movable plate and is connected to the other movable plate by cylinder and piston means for moving the movable frame relative to the stationary frame and therewith the mold parts between open and closed positions. The unit includes a clamping device for clamping the unit in fixed position on the rods and hydraulically operated means cooperating with the pressure plate for preventing the mold parts, when in the closed position, from opening under the pressure of the material injected thereto.
Injection molding machines usually have a split mold of which one part is usually mounted on a fixed plate whereas the other part is mounted on a plate movable toward and away from the fixed plate. Cylinder and piston means are provided for moving the two mold parts between an open and a closed position. The material from which the molded article is to be formed is fed under pressure into the cavity of the mold when the two mold parts are in closed position and the pressure at which the molded material is injected into the mold cavity tends to open the split mold. In order to avoid such an opening of the split mold during the molding process, a force must be provided acting against the force created by the injected material and this counteracting force evidently must be at least as great as the force created by the injected material. The force created by the injected material is considerably greater than the force necessary for moving the mold halves to the closed position. In order to be able to move the two mold halves quickly between the open and the closed position, the cross-section of the cylinder provided for this purpose is held as small as possible and such cylinder and piston means will not provide the necessary force for maintaining the mold halves in the closed position under the pressure of the material injected thereinto. Therefore, in many injection molding machines an additional cylinder with a greater cross-section and a piston therein is provided for creating the necessary large force for holding the mold halves in the closed position during the injection process.
The German Pat. No. 1,234,977 discloses an injection molding machine including a stationary frame having two parallel, spaced stationary plates between which a plurality of tubular members extend in which guide rods of a movable frame are guided for movement in axial direction, which carry on opposite ends thereof, projecting beyond the stationary plates, a pair of movable plates, one of which carries half of a split mold movable toward and away from the other half which is mounted on the stationary plate adjacent thereto. A guide plate is mounted on the rods between the other of the stationary plates and the movable plate adjacent thereto and abutting on an adjustable spindle turnably mounted on the other of the movable plates. A hydraulically operated knuckle joint is connected to the guide plate and the stationary plate adjacent thereto for moving the movable frame relative to the stationary frame and therewith the two mold parts between the open and the closed position, and the knuckle joint provides also the necessary force for holding the two mold parts in closed position against the pressure of the material injected thereinto. This known construction has the disadvantage that the movable frame has to extend in the region of the knuckle joint far beyond the stationary frame especially in the closed position of the two mold halves.
An injection molding machine is disclosed in the German Auslegeschrift 1,242,356 which differs from the above-described machine in that the knuckle joint is used only for moving the movable frame relative to the stationary frame and therewith the two mold halves between open and closed position, whereas the means for creating the pressure counteracting the pressure produced by the material injected into the closed mold comprises a fluid actuated membrane mounted in the guide plate and cooperating with a plate fixed at one end of the adjustable spindle. This known construction has evidently the same disadvantages as the construction described above and in addition the guide plate construction is more complicated.
A further injection molding machine is disclosed in the German Auslegeschrift 1,529,902 which differs from those mentioned above. In this machine a plate carrying half of a split mold is guided between guide plates extending between two stationary plates for movement toward and away from one of the stationary plates which carries the other half of the split mold. A frame likewise guided by the guide plates has a central axially movable wedge acting on a pair of clamping members for clamping the frame in fixed position to the guide plates. After the split mold is closed, the central wedge is moved in axial direction by another wedge extending normal to the central wedge and engaging an end face of the latter to move the clamping members to a clamping position and the other wedge acts also on the movable plate to provide the necessary force for holding the split mold in closed position against the pressure of the material injected thereinto. This construction has the disadvantage that the force preventing opening of the split mold during the molding operation is not independent from the clamping pressure of the clamping members and the additional disadvantage that the construction is rather complicated.